How close to the surface of the masonry can vertical reinforcing bars be placed in grouted collar joints?
The ACI 530/ASCE 5/TMS 402 code requires that reinforcing bars have at least 1/4 inch clearance for fine grout and 1/2 inch for coarse grout. This minimum requirement is to allow enough space for grout to flow around and embed the bars. I recommend leaving a minimum of 1/2 inch.